It's hard to know when it started - I think it can be quite insidious at first, and I guess DD was embarrassed and never said she had diarrhea. She didn't go often - only a couple of times a day - and didn't seem to have pain.
I noticed the toilet was often "spattery" and that one of the kids was obviously having loose poos, but wasn't sure who. Then DD had quite bad breath and she stopped growing and just didn't put on weight. My DH had been a skinny kid so we put it down to that, but then she stopped wanting to walk to school around Feb/March of year 6, having been so excited about it at the start of the year. She started really struggling to get up in the mornings, and was always tired. Her hair, which had been really pretty, looked dull and brittle. She would cry in the evenings just from being so tired. She was always a bit warm - her standard temperature was about 37.2.
Then while we were waiting for her appointment she started spiking random fevers - for 24 hours her temp would shoot up. Then she began vomiting randomly. She would get super breathless walking any distance. Also if we went out walking she would desperately need the loo.
She stopped eating almost everything except mashed potato with gravy, chicken nuggets, wotsits. Which is funny as now she actually has a very adventurous palate and loves spicy food.
It was a horrible time, looking back. I remember seeing her skipping along two weeks into the liquid diet and being so bloody grateful, as by the time she was diagnosed she could barely walk up a staircase.
After she was treated her "normal" temperature went down to 36 like everyone else. Now I think she was probably running a low fever the whole time.